Hitherto, as one of the methods of thermally cracking a heavy petroleum oil, a method has been proposed in which a gas, which does not react with the heavy petroleum oil, at a temperature of 400.degree.-2000.degree. C., is contacted with the heavy petroleum oil to thermally crack the heavy petroleum oil at a temperature of lower than 500.degree. C., thereby obtaining hydrocarbon gases, aliphatic hydrocarbon oils and aromatic hydrocarbon pitches (e.g., German Offenlegungsschrift No. 2 215 432). In concrete terms, the heavy petroleum oil is heated to a temperature of 450.degree.-520.degree. C. and the thus heated oil is introduced into reactors wherein the oil is contacted with the gas at a temperature of 400.degree.-2000.degree. C. to cause thermal cracking at a temperature of 400.degree.-440.degree. C.
However, according to the above mentioned method, when the heated heavy petroleum oil at a high temperature of 450.degree.-520.degree. C. is introduced into the reactors, an instantaneous contact occurs between the oil and the reactors, producing coke as a by-product and risking heat-shock rupture of the reactors.